The majority of commercial surgical gloves are still manufactured from natural rubber latex (NRL). However, NRL can lead to Type I allergic reactions, including risk of anaphylactic shock. In the race to reduce patient and medical staff allergy risks, a growing number of hospitals aim to eliminate products made from NRL, including surgical gloves. Drawbacks of NRL, including odor, and adverse allergic (Type I) reactions, led to the development of synthetic alternatives. However, replacement of NRL surgical gloves by synthetic alternatives has caused in the past some concerns regarding comfort and protection.
Needed today are surgical gloves with excellent properties in the following areas: tensile strength, modulus, and puncture resistance. The new surgical gloves should offer mechanical properties and protection that is at least comparable to NRL gloves. Moreover, the surgical gloves should preferably be superior to NRL gloves in terms of comfort (which is a balance of strength and modulus).
Similar demands are made with respect to other dip-moulded articles, also referred to as dipped goods, such as condoms.
Vulcanization is a chemical process for converting rubber or related polymers into more durable materials via the addition of a curing system such as sulphur or equivalent vulcanizers in combination with additives that modify the kinetics of the vulcanizer, known as accelerators. From WO 2013/025440 articles with enhanced tensile strength and comfort are known, that are prepared from a latex comprising water, a styrenic block copolymer and a vulcanizer, by a process which comprises coating a surface with the latex to obtain a film and vulcanizing the same. The styrenic block copolymer has 2 or more poly(vinyl aromatic) blocks and at least one block of polymerized conjugated diene, wherein the styrenic block copolymer has a weight average molecular weight of 150,000 to 250,000, the poly(vinyl aromatic) blocks have a weight average molecular weight ranging from 9,000 to 15,000, and the content of poly(vinyl aromatic) blocks in the styrenic block copolymer ranges from 8 to 15% wt., based on the total styrenic block copolymer. The preferred styrenic block copolymer that is used in the experiments is a linear styrenic block copolymer.
This reference also provides a latex comprising such a styrenic block copolymer and a vulcanizer, as well as a styrenic block copolymer that is particularly suitable for use in such a latex. Excellent mechanical properties are achieved with this latex/vulcanizer system. Moreover, the gloves prepared from the artificial latex excel in comfort. On the other hand, there is an increasing demand for dipped goods that are free of products used as vulcanizers and accelerators. Vulcanizers and accelerators on the one hand provide strength, but on the other hand may act as allergens, causing sensibilisation.
WO 2013025440 teaches to use a vulcanizer. The latex may in theory be used without vulcanizer. However, those that have tried to use the latex of WO 2013025440 without vulcanizer found out that the strength is then insufficient unless this system is annealed at a temperature of between 100-130° C., preferably about 120° C. Annealing at a higher temperature causes damage to the film. Moreover, they found that the latex used without vulcanizer when annealed at the appropriate annealing temperature suffers from a phenomenon referred to as “ballooning”. Ballooning is a permanent deformation of the film. There is no solution provided in the prior art with respect of this problem.
In U.S. Pat. No. 5,500,469 an artificial latex comprising a stable aqueous colloidal dispersion of a preformed multiblock copolymer prepared using a sulfate of an ethoxylated phenol as a dispersing and stabilizing agent is described. This composition is particularly suitable for preparing articles such as gloves or condoms that are free of vulcanizers. The multiblock copolymer has the formula:A-B—Ym—(B-A)n wherein each A is independently a polymer block of an alkenyl aromatic hydrocarbon, the total A being at least 5 weight percent of the total weight of the polymer; wherein Y is the remnant of a multifunctional coupling agent; m is 0 or 1; n is an integer from 1 to 5, preferably 1 to 3, more preferably 1; and B comprises a polymer block of a conjugated diene. The polymer illustrated in U.S. Pat. No. 5,500,469 is an SIS (styrene-isoprene-styrene) block copolymer containing 18% styrene and 82% isoprene and having a weight average molecular weight of about 130,000. Unfortunately, this polymer does not provide the superior comfort known from WO 2013025440. In other words, although this reference identified a broad range of multiblock copolymers as suitable, it did not disclose specific block copolymers with excellent comfort that can be used without vulcanizer and that does not suffer from ballooning when annealed.
In U.S. Pat. No. 5,563,204 an aqueous dispersion is claimed which is capable of forming a free-standing, coherent, elastomeric, solid film which, after drying and annealing at 80° C. for 30 minutes, demonstrates a tensile strength of about 11.0 MPa or greater. It is suggested to use one or more block copolymer(s) corresponding to the formula A-B—Xm—(B-A)n, wherein each A polymer block consists essentially of a monovinylidene aromatic monomer, having a weight average molecular weight from about 8,000 to about 15,000 Daltons, each B polymer block consists essentially of a conjugated diene and, optionally, a monovinylidene aromatic monomer having a weight average molecular weight from about 30,000 to about 200,000 Daltons, X is the remnant of a multifunctional coupling agent, m is 0 or 1, and n is an integer from 1 to 5.
In example 1 of this reference a film of an SIS block copolymer is prepared and the tensile strength at break is tested. In example 2 of this reference a radial block copolymer is used. There is no improvement when replacing a linear block copolymer by a radial block copolymer. Ballooning at this temperature does not occur and is not discussed. The latter is not surprising, as the annealing is carried out at a relatively low temperature; a temperature that is too low to provide sufficient strength when using styrenic block copolymers with relatively small endblocks.
It may therefore be concluded that the prior art broadly describes latexes based on linear and branched block copolymers, but that there is no clear teaching how to prepare a vulcanizer-free latex with excellent comfort that can be annealed to provide sufficient strength without suffering from ballooning.
In US 2005020773 Improved adhesives are provided through the use of styrenic radial block copolymers, containing at least 40 wt. % diblock copolymers, the adhesives have improved adhesive properties and a reduced elastic behaviour under die-cutting conditions. No latexes are disclosed.
The inventor set out to find a styrenic block copolymer and a latex comprising water and said styrenic block copolymer that may be used without vulcanizer, and that combines superior comfort, softness and strength without suffering from ballooning.
Moreover, the dip-moulded articles produced therefrom may have to be sterilized by gamma ray irradiation prior to use. Sterilization is particularly important for medical applications (e.g., surgical gloves, tubing, etc.) and food-contact applications. This, however, is not without problems.
The problem to provide a dip-moulded article, particularly gloves for medical use and the like, which have excellent strength and wearing feeling and which is not deteriorated even when the article is sterilized by irradiation with gamma rays is known. This has been discussed in US 2010204397. In this prior art reference styrene/isoprene/styrene block copolymers have been used. According to this reference the use of a phenolic antioxidant added to the dispersion medium is not good enough (cf. Comparative Example 1). Rather, another antioxidant must be added into the rubber, i.e., during the emulsification of the rubber. It would thus appear that dip-moulded articles, like medical gloves and condoms and the like, cannot be made such that they can be sterilized with gamma ray irradiation unless one uses an antioxidant that is present in the rubber constituting the rubber latex, together with a different antioxidant having a melting point of 40° C. or higher that is present in the dispersion medium constituting the rubber latex. This is not ideal.
The current inventors therefore set out to find a latex, capable of being used without vulcanization additives that can be used to make a dip-moulded article that can be annealed without ballooning and that can be sterilized with gamma ray irradiation without loss of properties. This problem too has now been solved.